A drive assembly or station for mining apparatus such as a conveyor or a coal plough is required to drive one or more chains by way of a rotary drum. It is often desired for the drive assembly to drive the drum at a low speed very much below the normal operating speed so as to slacken or tension the previously locked chain or chains during installation or repair work for example. To this end, a drive assembly is known which employs an additional drive motor especially for driving the drum at this low speed.
A standard form of drive assembly has main gearing in a housing which is connected to the machine frame of a conveyor, and is driven from an electric motor through a fluid coupling. In the case of the drive assembly for a plough, a safety device in the form of a shear pin drum is often provided on the side of the gearbox remote from the conveyor and has shear pins which shear if the chain load becomes excessive.
When auxiliary gearing is required to drive the chain drum at a low speed, the construction described above must be extensively modified and two different types of drive assembly manufactured, one with and one without auxiliary gearing. In consequence both the manufacturer and the user have to incur the expense of holding extra stocks. Also, where the user needs to change from an assembly without auxiliary gearing to one with such gearing, or vice-versa, difficulties are encountered since these tasks have to be performed in dirty and badly lit conditions in restricted spaces underground.
U.S. Pat. Nos. 3,855,871 and 3,863,522 describe mining apparatus including a drum for driving a chain, a main motor for driving the drum at an operating speed by way of a transmission comprising a gearbox and a chain-tensioning device having an auxiliary motor for driving the drum at a reduced speed. The chain-tensioning device is secured to the gearbox and is in the form of a self-contained unit.